Optical measurement systems are frequently used to analyze biological and chemical substances. In e.g. confocal microscopes a fluorophor is added to the substance under test. A laser light is used to excite the fluorophor, and when it subsequently decays radiatively a camera can detect its position.
Light is also used to study physical, chemical and biological reactions, which typically occur on a femtosecond (fs) to nanosecond (ns) scale. This is conventionally done by having a pump laser and a fs probe laser, which is slightly delayed compared to the pump laser.
In order to analyze many different samples and substances the optical measurement system should preferably contain several laser wavelengths. This can be achieved by combining multiple single line width lasers and/or having a tunable light source.
Applications within the field span light sources all the way from the UV (10-400 nm) through the visible (400-800 nm) to the near-IR region (800-2500 nm). E.g. to analyze spectra of gaseous benzene it is often preferred to measure spectra from 210 nm to 300 nm, whereas e.g. to analyse wheat it is normally preferred to span from 750 nm to 2500 nm.
The preferred choice of light source varies for different applications. Some examples are thermal sources or Ti:sapphire lasers for near-IR wavelengths, and Ti:sapphire based non-collinearly phase matched optical parametric oscillators for visible and UV wavelengths. Within the last decade fiber-based systems have also been used to generate broad-band sources, examples include frequency combs spanning 530 nm to 2100 nm (e.g. FC1500-250-WG Optical Frequency Synthesizer from Menlo Systems) and super continuum (SC) sources spanning 400 nm to 2400 nm (e.g. SuperK EXR-15 from NKT Photonics A/S or WhiteLase SC400 from Fianium Ltd). Other examples include sub-nanosecond pulsed LEDs, such as e.g. the PLS series from PicoQuant, which have pulselengths down to 500 ps and can reach up to 80 μW output power in the visible range and around 1 μW in the UV. Here PicoQuant notes that 1 μW is still sufficient to use the source as an efficient fluorescence excitation source.
Another approach is to frequency double, triple or quadruple optical pulses to obtain shorter wavelengths. These processes are commonly referred to as second, third and fourth harmonic generation. For brevity, all these processes will in the following be described as frequency doubling, i.e. the term frequency doubling should be understood to include harmonic generation of any order.
Frequency doubling can be obtained by sending light pulses with a high intensity through a non-linear crystal. Inside the non-linear crystal some of the light photons combine to create light at the doubled frequency (and thus half the wavelength) whereas other parts of the light traverse the crystal without being doubled; see FIG. 1. Accordingly the output beam from the crystal will contain light at both the original frequency f1 and the doubled frequency 2f1. The amount of light at the different frequencies depends on the degree of phase matching between the photons at the fundamental and doubled frequency (i.e. f1 and 2f1). The phase matching again depends on the intensity, spectral content and angular dispersion of the incoming light, but also on the crystals material, length and how it is cut.
For un-dispersed incoming light, the path length inside the crystal where there is phase matching is inversely proportional to the bandwidth of the incoming light. Thus extremely short crystals have been used for frequency doubling broad band pulses, for example Szabo states that for frequency doubling a 50 fs beam at 496 nm the crystal length should be shorter than 0.07 mm if the light is un-dispersed (Broadband frequency doubler for femtosecond pulse, G. Szabo and Z Bor, Appl. Phys. B. 50, page 51-54, 1990, see first paragraph on page 51).
Frequency doubling in short non-linear crystals is enabled by increasing the light intensity, since the degree of frequency doubling generally increases with the intensity of the incoming light. However, high intensity light will often lead to degradation of the crystal and hence limit its lifetime. This is in particular the case for conversion to wavelengths in the UV region.
For broad band sources, it has been shown that angularly dispersing the light before the non-linear crystal enables obtaining phase matching over a wider bandwidth. E.g. the prior mentioned reference by Szabo showed that dispersing the beam on a grating prior to the non-linear crystal enables doubling 10 fs pulses at 496 nm in a 1 mm long crystal, see FIG. 2. For reference 10 fs pulses at 496 nm must have a bandwidth of at least 25 nm (Fourier transform limit).
The idea of dispersing the light before the crystal was experimentally demonstrated in a paper by Baum (Tunable sub-10-fs ultraviolet pulses generated by achromatic frequency doubling, Peter Baum, Stefan Lochbrunner, Eberhard Rielde, Optics Letters, vol. 29, no. 14, Jul. 15, 2004, page 1686-1688). Here a set of prisms between the laser and the doubling crystal are used to enhance the doubling bandwidth by a factor of 80 and obtain a tunable source from 275-375 nm with <10 fs pulse length and a 360 μm thick BBO crystal, see FIG. 3.
A similar approach has furthermore been used to demonstrate a tunable source from 460 nm to 900 nm with <50 fs pulse length (Generation of 10 to 50 fs pulses through all of the visible and the NIR, Appl. Phys. B, 457-465, 2000, E Riedle, M. Beutter, S. Lochbrunner, J. Piel, S. Schenkl, S. Spörlein, W. Zinth).
Such a system is well suited for experiments requiring tunable very short fs pulses in the UV or visible range. However, it has a large cost, is complex to operate and requires highly skilled operators, and thus also has a large cost of ownership.
A lot of bio-optical applications require time resolved measurements but not necessarily on a short fs scale. Some examples include time resolved fluorescence, time correlation single photon counting, single molecule detection, intrinsic fluorescence, time resolved photoluminescence, UV polymerisation of resin, DNA sequencing, confocal microscope, FLIM, FRET, flow cytometry, cell-sorting, spectroscopy and food analysis.
Thus there is a commercial market for a low cost tunable light source spanning the UV-range with an output of at least 1 μW. The addressable market further increases if the tunability can be extended into the visible and/or near-IR wavelengths.